The present invention relates to a device for controlling a gas flow and more specifically to a variable geometry turbine for a turbocharging unit for an internal combustion engine. The invention further relates to application of the device in an exhaust aftertreatment system for controlling the function of an exhaust aftertreatment unit in the exhaust aftertreatment system. One application is to achieve a high engine braking performance.
Turbochargers are well known and widely used with internal combustion engines for purpose of increasing power output, decreasing fuel consumption and emissions, and compensating for air density loss at high altitudes. Generally, turbochargers supply an increased charge air supply for the combustion process than can otherwise be induced through natural aspiration by utilizing exhaust gas energy to drive an air compressor. This increased air supply allows more fuel to be burned, thereby increasing power and output not otherwise obtainable from an engine having a given cylinder displacement under natural aspiration conditions.
Variable geometry turbochargers (VGTs) allow the intake airflow to be controlled and thereby optimized over a range of engine speeds. A VGT may for this purpose be provided with a plurality of inlet guide vanes on the turbine stator. An inlet passage to the turbine has a circumferential extension around the turbine and forms an annular passageway. The inlet guide vanes on the turbine stator are arranged circumferentially spaced in this passage. The intake airflow is optimized by changing the angle of the inlet guide vanes on the turbine stator. An optimal position for the inlet guide vanes is determined from a combination of desired torque response, fuel economy, and emission requirement.
More specifically, the annular passageway is connecting a scroll shaped volute defined in a turbine housing to a turbine chamber where the turbine is located. Each vane is connected to a vane pin housed in a nozzle ring. The vane pin is connected to a vane arm which connects the vane pin with a unison ring. Pivotal movement of the unison ring enables simultaneous pivoting of the vanes in the annular passageway. In order to control the end positions of the vanes, in particular when the vanes are set to delimit a narrow gap in between the tips of the vanes, a stop screw is used.
Further, in order to handle the strict emission regulations today, an exhaust aftertreatment system (EATS) comprising a diesel particulate filter (DPF) and a selective catalytic reduction (SCR) system is often used. When the engine is running at low load, the exhaust temperature isn't always high enough for this EATS to function with satisfaction since a certain temperature level is required for the chemical reactions. Hence, the exhaust temperature needs to be increased in order to achieve an acceptable performance from the EATS, so called heat mode or thermal management.
The type of VGT provided with inlet guide vanes as described above can be used in order to achieve an increased exhaust temperature in order to achieve an acceptable performance from the EATS. This is achieved by, in some operating points, closing the vanes to a “zero gap” position. Closing of the vanes may however result in damage to the vanes. The individual differencies in vane angle due to tolerances and the fact that the vanes should not be forced to close against each other due to stress and wear will make the leakage over the vanes, and hence boost, different for different individuals.
It is desirable to achieve a gas flow control device suitable for a turbo unit, which provides conditions for a robust control functionality.
According to a first aspect of the invention it is provided a device for controlling a gas flow through a passage, wherein the device comprises a plurality of pivotable gas flow control vanes, wherein the pivot axes of a first and a second adjacent vane are spaced so that a trailing edge of the first vane overlaps a leading edge of the second vane when said first and second adjacent vanes are positioned in a first mutual end state for substantially restricting said gas flow through said passage characterized in that the second vane comprises a recess with such a shape that the trailing edge of the first vane is at least partly received in the recess when said first and second adjacent vanes are positioned in said first mutual end state.
The design of the vanes creates conditions for achieving substantially the same leakage between two adjacent vanes when the vanes are in said first mutual end state (which represents a “closed” position) since the design allows for differencies in vane angle due to tolerances.
Further, by designing all vanes with such recesses, conditions are created for achieving a controlled leakage. More specifically, the leakage will be substantially the same over the complete extension of the vane arrangement, ie along the complete annular passageway in the turbocharger application providing for a robust and accurate control of the individual turbocharger unit.
Further, this solution creates conditions for that different individual turbo units will deliver substantially the same back pressure during operation in operating points with small gaps. Thus, the risk of different turbocharger units behaving differently is decreased.
According to a preferred embodiment, the first and second vanes are configured such that a surface of the trailing edge of the first vane facing the recess is positioned at a distance from an opposite surface of the recess when said first and second vanes are positioned in said first mutual end state. By defining an end state (representing a “closed” state) with a gap between the vanes, wear between the vanes during operation is reduced and thereby the life is increased. Preferably, the vanes are locked in this end state.
According to a further preferred embodiment, the recess of the second vane and the trailing edge of the first vane are configured for establishing a substantially constant gap between said opposite surfaces provided the vanes are within accepted tolerances when said first and second vanes are positioned in said first mutual end state. This design of the vanes will result in that substantially the same leakage between two adjacent vanes is achieved when the vanes are in said first mutual end state.
According to a second aspect of the invention it is provided an exhaust aftertreatment system for an internal combustion engine comprising at least one exhaust treatment device and a device according to above arranged upstream of the exhaust treatment device for achieving an elevated temperature in the exhaust gases by positioning said flow control vanes in said first mutual end state. Using the inventive device in the aftertreatment system creates conditions for achieving a robust performance from the EATS, a so called heat mode or thermal management.
According to a third aspect of the invention it is provided a system for propelling a vehicle comprising an internal combustion engine and a device according to above arranged in an exhaust line from the internal combustion engine for achieving a high exhaust back pressure when said flow control vanes are positioned in said first mutual end state. Using the inventive device in the propelling system creates conditions for achieving a robust performance with regard to engine braking.